I need to validate my idea for a webapp and show traction. Unfortunately its too complex to prototype on my own. Would a video explainer and demonstration be enough? I am thinking of creating a landing page with a video explaining the basic problem and solution. There I can use a "Sign Up NOW" call to action to seek conversions. This way I can gain metrics on whether or not it's being validated. What do you think?
First off, the landing page with a video explaining the problem and proposed solution is a great lean approach to getting initial validation prior to building the product.
If you can, mockup some lo-fi wireframes of your idea for the webapp in Invision or Axure. Then get some prospective users to test your idea.
As was already mentioned, the hard part is finding those initial users that are interested in your product.
Lastly, you say your solution is "too complex to prototype on my own." That's a red flag that you may be trying to do too much too soon. I would take a step back and really spend some time thinking about the simplest solution to the single problem you are trying to solve. If you can identify a single piece of your webapp that you can get out in the market and gain traction with, do that.
After that, you can start to build out the other features and have a customer base to solicit feedback from. I've seen buttons for future planned features in some apps that just pop up a window asking the user to click "Yes" if they want that feature in the product. That's a great input when trying to prioritize which features to build next.